How would I know if I have a hyperactive thyroid?

Your thyroid gland—which is located in the neck, near the voice box—is responsible for regulating your metabolism. If your thyroid is hyperactive, it basically runs in high gear, according to Martin Roginsky, M.D., an endocrinologist at Nassau County Medical Center in New York. There are four telltale signs: First, "you lose weight despite eating a great deal," Dr. Roginsky says. You can't tolerate heat—you always feel warm, you suffer from a rapid heartbeat or palpitations, and your eyes become irritated. "The latter could manifest as a bulging of the eyes or a staring look," Dr. Roginsky adds. Graves' disease, associated with the bulging-eyes symptom, is one cause of hyperthyroidism, he says, but it sometimes affects the eyes alone and not the thyroid. If you experience any of the four symptoms above, see your doctor, advises Dr. Roginsky. "Though not a very common disease, untreated hyperthyroidism can adversely affect the heart. The disease, he says, is more common in younger people and sometimes affects pregnant women—who, when the case is mild are simply monitored; in severe cases, they're given anti-thyroid medication.